My present invention relates to a microwave circuit with coplanar conductor strips.
An important application of such a circuit is in monolithic or hybrid microelectronics, such as for example interdigitated filters.
The electrical state of two coupled transmission lines functioning with transverse electromagnetic waves can be represented at any time by two propagation modes superimposed on each transmission line.
A first mode, called the even mode, is characterized by pairs of conductors with cophasal voltage and current values (E, I) in any cross-sectional plane of each line; a second mode, called the odd mode, is characterized by pairs of conductors whose voltage and current values (E, I) are in phase opposition in any such cross-sectional plane. In the case of a line produced by microstrip technology, i.e. constituted by conducting strips arranged on the same face of a dielectric substrate whose other face is covered by a conductive layer known as a ground plane, the phase velocity of one mode involving propagation between one conducting strip and the ground plane (even mode) cannot be equal to the phase velocity of a mode involving propagation between two adjacent conducting strips (odd mode). Thus, in the first case propagation takes place almost exclusively through the substrate and in the second case a significant fraction of the energy is propagated in air, so that the very presence of the substrate makes the propagation medium nonhomogeneous. As a result, the equivalent electrical diagrams of the coupled lines become very complex or can but roughly approximate the actual conditions.
Various solutions have been proposed for homogenizing the dielectric and consequently reducing the variation between the phase velocities of the different propagation modes.
The Podell coupler described in Microwaves Magazine, March 1974, pp. 58 to 62 decreases the phase velocity in the odd mode without modifying the phase velocity in the even mode. However, this type of coupler cannot be used for coupling above 8 dB.
The overlay coupler described in the publication of IEEE Transactions on M.T.T., vol. 18, No. 4, April 1970, pp. 222 to 228 requires a transfer of dielectric material to the circuit and is difficult to realize, owing to the complicated procedure required for identically repeating the technical characteristics of couplers of this type.
Another solution, described by B. Schiek, J. Kohler and W. Shilz in the report of the 6th European Microwave Conference of 14-17 September 1976, published by Microwave Exhibitions and Publishers, Ltd., Temple Gate House, 36 High Street, Sevenoaks, Kent, TN 13 15G, England, comprises compensating the phase-velocity divergence of the even and odd modes by a stepped line construction having two coupled line sections with substantially the same length but different coupling factors due to their different spacings. This solution has the disadvantage of requiring complex calculations for its realization and, furthermore, does not make it possible to obtain, in its application to a Schiffman cell, an operation of the device on a frequency band higher than one octave.